The invention relates to a multi-section laser diode that can be switched between different wavelengths, more especially to a laser system comprising a control circuit that enables a multi-section laser diode to be switched rapidly between different wavelengths.
The original multisection diode laser is a three-section tunable distributed Bragg reflector (DBR) laser. Other types of multisection diode lasers are the sampled grating DBR (SG-DBR) and the superstructure sampled DBR (SSG-DBR) which both have four sections. A further multisection diode laser is the grating-assisted coupler with rear sampled or superstructure grating reflector (GCSR), which also has four sections. A review of such lasers is given in reference [1].
FIG. 1 is a basic schematic drawing of a SG-DBR 10. The laser comprises back and front reflector sections 2 and 8 with an intervening gain or active section 6 and phase section 4. An antireflection coating 9 is usually provided on the front and/or rear facet of the chip to avoid facet modes. The back and front reflectors take the form of sampled Bragg gratings 3 and 5. The pitch of the gratings of the back and front reflectors is slightly different, to provide a Vernier tuning effect through varying the current supplied to these sections. The optical path length of the cavity can also be tuned with the phase section, for example by refractive index changes induced by varying the carrier density in this section. A more detailed description of the SG-DBR and other tunable multi-section diode lasers can be found elsewhere [1].
Multisection diode lasers are useful in wavelength division multiplexed (WDM) systems. Example applications are as transmitter sources, as wavelength converters in optical cross connects (OXCs) and for reference sources in heterodyne receivers. Typically, WDM systems have channel spacings conforming to the International Telecommunications Union (ITU) standard G692, which has a fixed point at 193.1 THz and inter-channel spacings at an integer multiple of 50 GHz or 100 GHz. An example dense WDM (DWDM) system could have a 50 GHz channel spacing and range from 191 THz to 196 THz (1525-1560 nm).
The raison d'être of multisection diode lasers is their wavelength tunability. Each section of the laser diode is supplied with a drive current, and the lasing wavelength is a function of the set of drive currents, this function generally being quite complex. Setting the output wavelength of such a laser is thus usually performed by a sophisticated microprocessor controlled control system. As well as the fact that there is a complex relation between output wavelength and the set of drive currents, there is the additional factor that wavelength switching of the laser destroys its thermal equilibrium, which results in transient wavelength instabilities until thermal equilibrium is reached at the new set of drive currents. The time needed for temperature stabilisation can be quite long.
The transient thermal properties consist of two main effects.
A first effect is that, directly after the laser is switched, the thermal gradient across the device to the heat sink upon which it is mounted will be different to that measured at steady state operating conditions for these currents, due to a different heating level generated in the laser as the currents are different. This steady state temperature gradient will reassert itself over a period measured in a timescale from a few hundred nanoseconds to tens of microseconds. Because the device is at a different temperature during this period some temperature tuning of the wavelength occurs. For a positive (negative) change in tuning current the change in temperature will be such that the device is initially colder (hotter) than at equilibrium for those currents and some time will pass before the extra current dissipates enough heat energy to change this. During that period the device will be colder (hotter) than expected so a blue (red) shift from the expected output wavelength will occur.
A second effect takes place over a much longer timescale. The laser is thermally connected to a heat sink of finite thermal mass which has a temperature controller maintaining its temperature. The temperature controller cannot react instantaneously to a change in temperature, which means that with an increase (decrease) in bias current, the heat sink will heat up (down). This in turn means that for a given temperature gradient the device will have a different temperature, because the temperature the gradient is referenced from will be different. This temperature change results in the temperature of the device overshooting and going higher (lower) than would be normal for those currents. This effect will persist until the temperature controller returns the heat sink to its normal temperature, which may take 1-1.5 seconds.
To overcome the problems associated with transient (and non-transient) thermal effects, and also any other effects that cause the wavelength to deviate from the intended wavelength for a predetermined set of drive currents, a wavelength measuring system can be included which supplies measurements of the output wavelength to the control system. The laser drive current can then be adjusted in a feedback loop to provide locking of the output to the desired output wavelength.
FIG. 2 shows a typical application example where a SG-DBR laser is used as a source for a WDM system, with a microprocessor control system being provided for wavelength locking.
A SG-DBR 10 has a pigtailed output connection to an optical fibre 20. An optical coupler 12 is arranged in the optical fibre output path 20 to couple off a small proportion of the output power, for example 5%. The coupler 12 may be a fused taper coupler, for example. The part of the output beam diverted off by the coupler 12 is supplied to an optical wavelength locker 14, for example a JDS Uniphase WL5000 Series Wavelength Locker. The optical wavelength locker 14 is a wavelength measuring device based on a Fabry-Perot etalon. For WDM applications, the etalon is designed to have its cyclical frequency response matched to the ITU grid.
FIG. 3 shows the frequency response of the etalon in terms of its percentage throughput T as a function of frequency f. The frequency response of the etalon is such that an ITU channel frequency occurs on the maximum positive slope of the etalon peaks, as indicated in the figure. The optical wavelength locker 14 includes first and second photodiodes PD1 and PD2. Photodiode PD1 is arranged to receive light transmitted by the etalon. Accordingly, with reference to FIG. 3, if the output frequency of the laser is, for example, greater than the ITU frequency, the photodiode PD1 will receive a higher incident power level P1 than it would at the ITU channel frequency. Similarly, if the output frequency of the laser is below the ITU channel frequency, the power P1 incident on the photodiode PD1 will be lower than the value it would have if the laser output was at the ITU channel frequency. The photodiode PD1 thus outputs a voltage Vpd1 that can be used as a basis for generating an error signal relating to the frequency deviation of the laser output from the ITU channel frequency.
The second photodiode PD2 of the optical wavelength locker is arranged to measure the optical power input to the locker 14, thereby providing a measure of the total output power of the laser in the form of a measurement voltage Vpd2. The measurement voltages Vpd1 and Vpd2 are supplied by respective signal lines 16 and 18 to an analogue-to-digital converter (ADC) 22. The ADC 22 may for example have 12 bit resolution. The ADC 22 supplies the digitised measurement voltages Vpd1 and Vpd2 to a microprocessor 24 which may be connected to ancillary computer equipment through an interface 26.
When initially setting the laser 10 to a given ITU channel frequency, the microprocessor 24 refers to a predetermined set of drive voltages Vf Vb Vg and Vph for the ITU channel frequency concerned. The sets of drive voltages may be conveniently held in a look-up table, for example. The microprocessor 24 may thus include on-chip memory for this purpose, for example flash memory. To set the laser 10 to a particular ITU channel frequency, the microprocessor 24 asserts a set of voltages to a digital-to-analogue converter (DAC) 28. The DAC 28 may have 12 bit resolution, for example. The DAC 28 then supplies these voltages to a driver circuit 30 which converts the voltages to corresponding drive currents If Ib Ig and Iph which are then applied to the front reflector, back reflector, gain and phase sections 8, 2, 6 and 4 respectively of the SG-DBR 10.
Feedback from the optical wavelength locker 14 is provided in this control system by the microprocessor 24 continually re-adjusting the set of voltages sent to the DAC 28 on the basis of the measured voltages Vpd1 and Vpd2. The feedback adjustment is implemented principally through varying Iph, the current applied to the phase section 4 of the SG-DBR 10. The manner in which this is performed is now described. First of all, however, it is noted that, although the active wavelength control of the laser 10 is effected primarily through adjusting the phase current, adjusting the phase current will generally have other consequential effects, such as causing changes in the cavity loss. These can be compensated for by adjusting the gain current Ig. (Alternatively, compensation may be achieved using an external variable optical attenuator (VOA) arranged in the output path 20 after the coupler 12.) Consequently, although the wavelength control is principally implemented by varying the phase current, the gain current and possibly either of the other control currents may be changed as part of the feedback. For the sake of simplicity, the following description refers only to variance of the phase current.
The phase current Iph is varied by a correction factor Ierr defined by the following equation
      I    err    =      k    ⁡          (                                    V                          pd              ⁢                                                          ⁢              1                                            V                          pd              ⁢                                                          ⁢              2                                      -                  R          ITU                    )      where Vpd1 and Vpd2 are voltages proportional to the powers P1 and P2, as described above, RITU is the value of Vpd1/Vpd2 at an ITU channel frequency, and k is a constant factor. Generally a separate value for RITU will be used for each ITU channel, these values being stored in a look-up table, which may form part of on-chip memory of the controlling microprocessor, or may be held in an EPROM or other memory. The values of RITU will typically be preset during a calibration performed at the manufacturing stage. Correction of the phase current, by setting Vph→Vph−Verr in each control cycle, is effective since the error current Ierr is proportional to the wavelength deviation from the ITU channel wavelength. Thus, if the value of Verr is negative, the phase current is increased by a small amount, and vice versa. The procedure repeated until the different between the measured value and the stored value is within a tolerance. The phase current is thus used to provide fine tuning of the output frequency of the laser, with increases in phase current typically causing increases in the output frequency of the laser.
A conventional control system for wavelength locking such as that described above, or in reference [2], is thus based on calculating an error factor from the deviation of the ratio P1/P2 from a desired value of P1/P2 for the wavelength channel concerned, stored as the compound ratio value RITU.
The control loop is thus dependent on performing a division operation. Division operations can be easily performed using a microprocessor, such as a digital signal processor (DSP), and can also be performed by certain types of multiplier components. However, microprocessor and multiplier chip implementations both have limitations.
A drawback of using DSP or other microprocessor chips is that an analogue-to-digital (A-D) sample must be made at the input, and a digital-to-analogue (D-A) output must be made at the output. This takes some time to perform and limits the locking speed of the system.
A drawback of using multiplier chips is their accuracy and bandwidth. The accuracy is typically worse than ±2% and the bandwidth will be limited to a maximum of about 1 MHz. This limits the speed and accuracy of the locking mechanism.
With the prior art control system using microprocessor chips, or with multiplier chips that allow divide operations, it should be possible to improve the switching speed beyond the tens of millisecond range, perhaps up to as fast as tens of microseconds. However, at least with present commercially available electronic components, it is not possible to attain faster switching times.
However, ideally, the control system should have a response time approaching the fundamental limit of the switching time of a diode laser, which is of the order of ten nanoseconds.